The expression "there's no such thing as a free lunch" means a. if one person gains, someone else must lose. b. each person must pay for exactly what he or she receives. c. when scarce resources are used to produce one good they are unavailable to produce others. d. you cannot have a free lunch at the expense of someone else.

b. each person must pay for exactly what he or she receives.

Explanation:

The expression "There's no such thing as a free lunch" means that each person must pay for exactly what he or she receives.

This phrase is a common dialogue by the people to suggest that there is no such thing as any free stuff. If a person wants anything, he/she has to pay or give something in return for the thing s/he's going to get. It doesn't necessarily mean the literal 'lunch'. It's just a common way of referring to the overall scheme of things that nothing is ever free in this world.  

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